Technical Field
The present disclosure relates generally to compositions for high emissivity coatings that can be applied to various types of substrates, and to processes for manufacturing high emissivity coating compositions. More particularly, the present disclosure relates to (1) high emissivity coating compositions with enhanced strength properties, (2) high emissivity coating compositions in which titanium dioxide (TiO2) is used an emissivity enhancing agent; and (3) manufacturing processes for cost-effectively producing high emissivity coating compositions using an industrial waste source or stream.
Description of the Related Art
Increasing demand for energy and rising energy costs throughout the world have inevitably increased the need for energy users to save or conserve energy, particularly among industrial entities. In many instances, industries that use fired heaters or furnaces, such as refineries and petrochemical complexes, have attempted to maximize fired heater efficiency to thereby reduce fuel consumption. High emissivity coating technology has become a proven means for various high temperature applications to effectively increase radiant heat transfer and save energy without compromising process reliability and safe operation.
Emissivity (symbolically represented as c or e) can be broadly defined as the relative ability of a surface to emit energy by radiation. More particularly, emissivity can be defined as the ratio of energy radiated by a particular material to energy radiated by a blackbody at the same temperature. Higher emissivity corresponds to an increase in thermal efficiency. An increase in thermal efficiency attributed to high emissivity coatings in high temperature fired heater or furnace applications results in an increase in fired heater or furnace performance or output and/or a decrease in fuel consumption and overall energy demand.
The benefits and advantages of high emissivity coatings have led to various research and development efforts over the years to improve the performance of high emissivity coatings. In particular, research has been conducted to develop emissivity coating compositions that include emissivity enhancing agents (“emissivity agents”) to a) enhance emissivity values in order to increase radiant heat transfer; b) improve coating adhesion on substrates; c) extend coating lifetime across multiple high temperature cycles; and d) reduce emissivity agent degradation.
Currently, several high emissivity coating compositions are commercially available. Emissivity agents in such compositions can be derived from various sources. One commonly used emissivity agent is silicon carbide (SiC), which can exhibit good emissivity enhancement performance up to moderate temperatures. However, the use of SiC as an emissivity agent in applications involving high operating temperatures (e.g., fire heater, furnace, preheater, reformer, other refractory applications, or aerospace applications) can lead to a substantial decrease in emissivity and mechanical strength of the coating composition over time, and hence an overall decrease or degradation in the performance or function of a coating composition relying upon SiC as an emissivity agent.
In NASA Technical Memorandum 130952, entitled “Thermal Degradation Study of Silicon Carbide Threads Developed for Advanced Flexible Thermal Protection Systems,” published in August 1992, H. K. Tran and P. M. Sawko found that a surface transition of SiC to SiO2 was observed at temperatures greater than 400° C. The surface transition of SiC to SiO2 was due to temperature induced decomposition of SiC bonds and the subsequent formation of SiO2. The decomposition reaction of SiC at high temperature can be illustrated as follows:SiC+O2→SiO2+CO2 
Such decomposition of SiC can undesirably result in significant material shrinkage, unintended SiO2 passivation, and shorter coating composition lifetime. Accordingly, a need exists for improved emissivity agents for high emissivity coating compositions, particularly with respect to improving emissivity agent performance of emissivity coatings at high temperatures. Unfortunately, prior research on high emissivity coating compositions has failed to adequately consider or recognize that certain substances can potentially have a significant impact on enhancing emissivity values.
In addition to the foregoing, although aspects of high emissivity coating performance are being investigated and improved, existing processes for manufacturing high emissivity coating compositions fail to appropriately consider or address certain economic aspects of producing such compositions. In particular, while various efforts have been made to develop high emissivity coatings with better performance, such efforts have largely ignored or unavoidably increased the cost of producing such coating compositions and the end price of high emissivity coating products. Accordingly, there is a need for preparing, manufacturing, or formulating high emissivity coating compositions in a more cost-effective manner.